I used one for screwmounts years ago, and eventually sold it. It's a cropping finder like the VIOOH/IMARECT, which means the view gradually gets smaller as you go through the focal lengths. 35mm isn't bad, but 135mm is like framing through a tiny hole. And in case that description doesn't make it obvious, there's no framelines - just a rectangular aperture.
This does mean that you shouldn't have any issues with regards to not being able to see the full frame with glasses on, but from what I remember, the eyepiece is metal with a serrated edge, so it's a pretty easy way of leaving deep scratches in your lenses unless you cover the metal with something.
Basically, if I was you, I'd give it a miss. You're better off with dedicated finders for your favourite focal lengths if you're not happy with the built-in M finder.